A liquid crystal display apparatus which adopts an active matrix system is adopted widely, for example, in liquid crystal projector apparatus, liquid crystal display apparatus and so forth.
In such a liquid crystal display apparatus according to an active matrix system as just described, pixel cell driving circuits each including a pixel switch formed from, for example, a MOS transistor and a pixel capacitor connected to the pixel switch are formed, for example, on a semiconductor substrate in such a manner that they are arranged in a matrix. In short, a plurality of scanning lines are disposed along a horizontal (row) direction while a plurality of data lines are disposed in a vertical (column) direction. Further, a pixel cell driving circuit is connected at a position corresponding to each of intersecting points between the scanning lines and the data lines. Further, an opposing substrate on which common electrodes are formed is disposed in an opposing relationship to the semiconductor substrate, and liquid crystal is enclosed between the semiconductor substrate and the opposing substrate. Such a structure as just described is used to form a liquid crystal display apparatus.
Driving for image display of such a liquid crystal display apparatus as described above is described below briefly.
To the scanning lines disposed in the horizontal direction, a voltage of a predetermined level is successively applied, for example, for each one horizontal scanning period. In short, sequential scanning of the scanning lines is performed. In this instance, a plurality of pixel switches connected to a scanning line being scanned are placed into an on-state. Simultaneously, the data lines are driven within one horizontal scanning period. In short, a voltage based on data is applied to each of the data lines. It is to be noted that, thereupon, data line driving according to a so-called sequential driving system wherein data are successively applied to the data lines is usually performed.
The data applied in this manner are accumulated as charge through those pixel switches, which are placed in an on-state in such a manner as described above, as charge into the corresponding pixel capacitors. In short, writing of data into pixel cells for one horizontal line is performed. After writing of data is performed in this manner, a potential difference appears between the charge accumulated in each of the pixel capacitors and a Vcom applied to a corresponding opposing electrode, and the liquid crystal enclosed between the pixel capacitor and the opposing electrode is excited by the potential difference. In short, driving of the pixel cell is performed.
Such driving of the pixel cells for each one scanning line is executed each time the scanning lines are successively scanned thereby to display an image, for example, for one screen.
Further, in display driving of a liquid crystal display apparatus, driving is usually performed in such a manner that deterioration of the liquid crystal by application of a dc voltage to the liquid crystal is prevented. One of such ac driving systems, polarity reversal driving is known wherein pixel data is reversed to the positive polarity side and the negative polarity side with respect to the Vcom to drive each pixel. For the timing of the reversal driving, a frame reversal method wherein the polarity is reversed in a unit of a frame, a line reversal method wherein the polarity is reversed for each horizontal line, a dot reversal method wherein the polarity is reversed for each pixel cell (dot) and so forth are available.
Incidentally, in recent years, increase in definition and miniaturization of a liquid crystal display apparatus have been and are being promoted. In this instance, however, since the number of pixels per unit area increases, it cannot be avoided that the period of time permitted to write data signal into a pixel capacity becomes short. Therefore, such a problem that insufficiency of the gradation, color shading, degradation of the color reproducibility and so forth occur is liable to occur from a failure to write a data signal up to a necessary potential in the permitted period of time.
In order to eliminate such a problem as just described, it is necessary to raise the driving rate higher than ever. To this end, for example, not only increase of the number of scanning lines to be scanned or driven at a time but also, also in this instance, application of a higher gate voltage to each pixel switch can be listed as a countermeasure. According to this, refreshment of each pixel is performed at a higher rate.
However, the signal of image data applied to each data line varies at a predetermined timing within a range between a predetermined positive polarity maximum amplitude level and a predetermined negative polarity maximum amplitude level around the Vcom as described hereinabove. For example, a pixel switch is in most cases formed from a transistor of the N channel type or the P channel type, and in such an instance, in order to prevent a drop of the writing rate of a data signal by an increase of the on-resistance, a gate voltage higher than the amplitude of the image data signal must be applied.
Then, if a gate voltage of a further high level is applied from the circumstances described above, then specifications of a semiconductor process wherein transistors of a higher voltage withstanding property are formed must be employed.
For example, if the number of pixels per unit area is increased in order to raise the definition of or miniaturize a liquid crystal display apparatus, then the size of the individual pixel cells becomes smaller, and consequently, also the size of, for example, each pixel switch becomes smaller. However, from the characteristic of a semiconductor process, as the size of a transistor decreases, the voltage withstanding property of the transistor is obliged to decrease.
On the other hand, if it is intended to increase the voltage withstanding property in a semiconductor process, then the size of elements such as transistors is obliged to increase. Therefore, it becomes rather difficult to achieve such increase in definition and miniaturization of a liquid crystal display apparatus as described above from a reason that such increase of the size as mentioned above makes it difficult to assure a pixel capacitor or the like. In short, the miniaturization and the increase of the withstanding voltage have a mutually contradictory relationship. Further, since also modification to specifications in a semiconductor process from current specifications is involved, the increase of the withstanding voltage is inferior also in terms of the cost.
However, if a CMOS configuration is adopted for the pixel switches, then the gate withstanding voltage may be set to a gate withstanding property higher than the signal amplitude of the positive polarity or the negative polarity. Also in this instance, however, since the size of the transistors in the form of CMOS transistors becomes large, it is difficult to achieve increase of the definition and miniaturization, and a high cost is required similarly. Further, particularly since the junction capacitance of the pixel switches connected to the scanning lines and the data lines increase, it becomes difficult to raise the speed of data writing into the pixel capacitors.
Further, where a liquid crystal display apparatus adopts transistors of the N channel type or the P channel type for the pixel switches, the gate threshold voltage is raised by a so-called back bias effect. Therefore, even if a certain prescribed gate voltage is applied, the range of the effective gate voltage is narrowed by the gate threshold voltage raised as described above. If the liquid crystal is driven with the gate voltage whose range is reduced in this manner, then also the range of the reaction of the liquid crystal with the driving voltage level is narrowed, and also the gradation representing property is deteriorated as much.
Thus, as one of methods of eliminating such problems with regard to the characteristics as a liquid crystal display apparatus as described above, it is a possible idea to apply a higher gate voltage to the pixel switches as described hereinabove. However, if it is tried to realize this, then problems in a semiconductor process arise similarly as described hereinabove.
Where such problems as described above are taken into consideration, it is desirable to maintain the withstanding voltage of the transistors of the pixels switches as standards in a semiconductor process and besides make it possible to perform display driving which allows, for example, application of a higher gate voltage than ever.